The present invention relates to optical communications and, more particularly, to a symmetric coherent orthogonal frequency-division multiple access-passive optical network (OFDMA-PON) configuration with low speed optical network unit-side ONU-SIDE.
Orthogonal frequency-division multiplexing (OFDM) based passive optical network (PON) has emerged as an attractive solution and a candidate technology for next-generation fiber-optic access systems. Due to the strict cost constraints in such systems, especially from the optical network unit (ONU) side, direct photodetection has been employed to simplify optical hardware. However, in future PON, transmission speeds are expected to grow to 40+ Gb/s, while transmission distances and required ONU counts per fiber are also expected to at least double. Consequently, direct (non-coherent) photodetection will likely impose sensitivity limits on ONU-side optical signal reception that may render it unsuitable for certain future PON applications. Moreover, in the context of OFDM-PON ONUs, achieving 40+ Gb/s speeds with direct photodetection requires high-speed analog-to-digital and digital-to-analog (ADC/DAC) components, as well as high-speed digital signal processors (DSP). For certain applications, the high-speed ADC/DAC and digital signal processing DSP requirements can prohibitively increase both ONU cost and power consumption.
Various flavors of wavelength division multiplexed (WDM)-PON have been proposed to solve the problems described above. In classical WDM-PON, the problem is addressed by replacing passive splitters in the fiber network with WDM multiplexers and demultiplexers, and assigning each user a dedicated wavelength pair λi, DS/λi, US for downstream/upstream transmission. However, this approach would require dramatic and expensive changes to the deployed fiber distribution network. Moreover, the transmission speed on each wavelength would be limited to 1.25 Gb/s and the approach would prevent statistical bandwidth sharing to reduce wasted bandwidth resources.
A second type of WDM-PON that has been proposed removes the need for WDM multiplexer/demultiplexer installation by exploiting an ONU-side tunable laser to lock onto the upstream wavelength λi, US based on the downstream wavelength reference λi, DS that is pre-assigned at the optical line terminal (OLT). This tuning and locking feature is known already. In this way, colorless WDM operation is achieved without the need for reflective ONU-side optical devices or a fiber distribution network overhaul. Moreover, the use of the ONU-side tunable laser enables ONU-side coherent detection, which increases optical receiver sensitivity. However, to enable low speed ADC/DACs and DSP, this configuration also limits the transmission speed on each wavelength and prevents statistical bandwidth sharing among different ONUs in the PON. Consequently, speed limitations and bandwidth inefficiency are not solved by this approach.
Finally, in applicant's previous work, a source-free configuration with OLT-side coherent detection has been proposed for 40+ Gb/s/λ transmission with increased sensitivity. However, in this architecture, either high-speed ADC/DAC/DSP and/or expensive radio frequency (RF) clock sources were needed. Moreover, in the case of RF clock source use, different ONUs would need to have different pre-assigned clock frequencies. Consequently, the ONUs were not truly colorless (i.e. frequency independent) in the electronic domain, which is highly impractical from the deployment and management perspectives.
Accordingly, there is a need for a solution, that overcomes the above problems and challenges, wherein the receiver sensitivity limits and high-speed ADC/DAC and DSP requirements mandated by direct photodetection would thus be valuable for future OFDM-based PON, both from performance and cost perspectives.